1. Field of the Invention
The present invention relates to a phase-locked loop (PLL) system, and more particularly, to a PLL system which can generate a signal that automatically oscillates in a target frequency with an optimal state by controlling a band selection, a gain tuning, and a phase locking of a voltage-controlled oscillator (VCO), and a method of using the PLL system.
2. Description of Related Art
FIG. 1 is a block diagram illustrating a conventional phase-locked loop (PLL) system 100. Referring to FIG. 1, the PLL system 100 includes a phase/frequency detector (PFD) 110, a charge pump 120, a loop filter 130, a voltage-controlled oscillator (VCO) 140, and a frequency divider 150. The PLL system 100 receives a reference clock signal REF, and generates an oscillation signal LO which is phase-locked with the reference clock signal REF. In this case, the oscillation signal LO has a frequency that is acquired by multiplying a frequency of the reference clock signal REF by N times.
The conventional PLL system 100 functions to multiply the reference clock signal REF by a predetermined division ratio. Also, the conventional PLL system is widely utilized for generating a clock signal and recovering a clock and data.
The mismatch between the simulation and measurement in designing VCO resulting from process variations requires wide band VCOs which can sufficiently compensate the process variations. Also, to support multi-band and multi-mode wireless communication standards, it is indispensable to design wide band VCOs.
When designing the VCO 140 which oscillates in a broader frequency band, an operating frequency band of the VCO 140 is divided into several bands, so that gain features of the VCO 140 are not enormously enlarged. In this case, a band containing a target frequency must be selected from the several bands, so as to generate a frequency which is required in a system. However, when a user selects the band via a manual manipulation, it is very disadvantageous in aspects of time and costs. Accordingly, technologies for selecting a band via an automatic operation have been introduced.
Also, the gain features of the VCO 140 are not the same for each band. Accordingly, since a loop operating characteristic of the designed PLL circuit may be distorted, a gain adjustment function is required in addition to an automatic band selection function.